In the transmission of an audio signal, for example in radio transmission, cable transmission, satellite transmission and in recording devices, it is known to convert the analog audio signal to a digital audio signal with a certain resolution (definition), to transmit it in this form, and to re-convert it into an analog signal for reproduction. By means of the digital transmission, in particular, a better signal-to-noise ratio in reproduction is achieved.
The band width required for transmission of such a signal is mainly determined by the number of sampling values to be transmitted per time unit as well as by the resolution.
In practice, the objective is to keep the band width required for transmission as small as possible in order to be able to use a narrow band cable or to transmit as many audio signals as possible via an existing channel at the same time. The required band width can be reduced as such through a reduction of sampling values or the number of bits per sampling value.
These measures usually cause deterioration in reproduction, however. In a known method (DE OS 35 06 912.0) for enhancement of reproduction quality, the digital audio signal is transformed in successive (in time) partitions or sections into a short-time spectrum which represents the spectral components of the signal for the respective time intervals, e.g. 20 ms. In the short-time spectrum, owing to psychoacoustical laws, such components which are not perceived by the listener, i.e. which are irrelevant for communications purposes, can be found more easily in general than in the time range. These components are less weighted or left out entirely in transmission. Through this measure a material part of the otherwise necessary data can be left out so that the average bit rate can be markedly reduced.
It has become apparent that in case of amplitude variations within such a partition or section, particularly in case of signals starting only inside the block out of quietness, these signals are superimposed by disturbances after transmission. The cause for the perceptibility lies in the fact that the disturbances are also present before the signals start and are therefore insufficiently masked.
These disturbances can be generated, for example, by quantization noise which superimposes itself upon the shorttime spectrum and in which also noise parts appear within the whole block in the time range after re-transformation. In order to reduce these disturbances, each block is partitioned in the sub-blocks and the jumps in amplitude of the signals from one sub-block to an adjacent sub-block are detected. In case of amplitude jumps of more than 20 dB, the signals in the preceding block are compressed and, after the re-transformation, expanded.
Here, the following difficulties may arise:
a) The detection of real jumps is uncertain. PA0 b) Only such changes in energy in prior determined sub-blocks which exceed a certain scale are evaluated as jumps. PA0 c) Due to uncertainties in the performed energy detection unintended signal exaggerations may appear in the elevated signal partition.